1. Field of the Invention
This invention relates to a process for manufacturing boron-doped GaAs single crystals of low dislocation density which are suitable for preparing a stable semi-insulating substrate.
2. Description of the Prior Art
Single crystals of gallium arsenide (GaAs) have come to be used over an increasing range of optoelectronic applications, including the manufacture of semiconductor lasers and GaAs IC's. It is, however, difficult to obtain large GaAs single crystals with low dislocation density, as opposed to silicon. Therefore, the doping of GaAs single crystals with boron has been proposed to provide GaAs single crystals of low dislocation density, as disclosed in Japanese patent application No. 148335/1981.
The recent development of direct synthesis methods and the LEC (Liquid Encapsulated Czochralski) method employing a boron nitride crucible have led to an extensive research for methods of preparing single crystals of undoped semi-insulating gallium arsenide, which is a compound semiconductor. A semi-insulating substance is generally meant as a substance having a resistivity between 10.sup.6 and 10.sup.9 .OMEGA. cm.
The single crystals of GaAs of high purity produced by the LEC method are commercially available in the form of &lt;100&gt; wafers having a diameter of two or three inches. The gallium arsenide produced by the LEC method has, however, a dislocation density of 10.sup.4 to 10.sup.5 cm.sup.-2 which is 10 to 100 times higher than that of GaAs obtained by the HB (Horizontal Bridgman) method. Since the effects which dislocation may have on the yield and stability of IC devices employing semi-insulating GaAs have been recently clarified, it is essential to use GaAs crystals of low dislocation density in order to obtain devices of higher stability. The electrical property with dislocations is different from that without dislocations, because the impurities are gettering at the neighborhood of the dislocations.
It is well known that (a) a low temperature gradient near the solid-liquid interface, (b) a horizontally flat or convex solid-liquid interface, and (c) necking-in technique are important to obtain GaAs crystals of low dislocation density. It is, however, very difficult to satisfy these requirements.
There is a report indicating that it is relatively easy to obtain GaAs of low dislocation density when it is doped with S or Te (Yasuo Seki et al., J. Appl. Phys., 49 (1978), p. 822 ff.), but semi-insulating GaAs can not be obtained by doping with S or Te. It is theoretically possible to expect a low dislocation density by doping GaAs with boron. According to the single-bond strength model proposed by Yasuo Seki et al., the bond energy between B and As is 72.9 kcal/mol and higher than that between Ga and As in the matrix (47.7 kcal/mol), and which ensures greater resistance to dislocation, since boron atoms resist dislocation, and they are considered to be electrically neutral in GaAs.
It has hitherto been conventional to melt a mixture of GaAs and BAs to produce boron-doped GaAs single cystals. Boron arsenide is, however, difficult to obtain and expensive, though boron is easy to obtain and is inexpensive. It is easier to obtain a prescribed quantity of boron in boron-doped single-crystal GaAs by mixing GaAs doped with a different quantity of boron, than by employing a specific quantity of BAs relative to GaAs. Boron is difficult to dissolve in GaAs, which presents a substantial problem with adding boron in a large quantity to GaAs.